01 - Boil linguine in a large pot of salted water until al dente. Reserve 1/2 cup of pasta water, then drain the pasta.
02 - Sprinkle shrimp with kosher salt and ground black pepper evenly.
03 - Heat olive oil in a large skillet over medium-high heat. Add shrimp in a single layer and cook 1 to 2 minutes per side until just opaque. Remove shrimp and set aside.
04 - Add minced garlic and red pepper flakes to the skillet. Cook for 30 seconds until fragrant.
05 - Pour in white wine or chicken broth, scraping the bottom of the skillet to release browned bits. Simmer for 2 minutes to reduce slightly.
06 - Add lemon zest and juice to the skillet, then stir in baby spinach. Cook until spinach wilts, about 1 minute.
07 - Return shrimp to the skillet. Add drained pasta and butter. Toss together, gradually adding reserved pasta water to achieve a smooth sauce.
08 - Remove from heat, sprinkle with chopped parsley, and serve immediately topped with Parmesan if desired.